Crown ethers are widely used because of their capability of complexing various cations. The extent of complex formation, complex stability constant, in case of a given cation depends apart from the geometrical characteristics of the crown ether, such as ring-member number, hetero atom number, on the solvent as well. The stoechiometry of the formed complex is influenced by the ratio of the diameter of the crown ether ring and the diameter of the cation. All the above factors justify the use of crown ethers as active ingredient of ion selective membrane-electrodes.
In the last 17 years various substances, inorganic precipitates, various ion exchanging compounds, electrically charged or not charged complex forming agents were used as ion-selective electrodes for measuring various anions (halides, pseudohalides, nitrates etc.) as well as various cations (alkali metal ions, alkali earth metal ions and some heavy metal ions etc.) (Karl Camman, Das Arbeiten mit ionenselektiven Elektroden, Springer Verlag, Berlin, Heidelberg, New York, 1977; Peter L. Bailey, Analysis with Ion-Selective Electrodes, Heyden, London, New York, Rheine, 1976). The various electrodes have different active ingredients, mechanical and dynamical properties and different so called selectivity factors, latter being the most important parameter from the point of view of the use.
In the most significant field of the practical application, i.e. in the field of biochemical applications those electrodes are especially important, which are suitable for monitoring the ionic processes of the cell metabolism in the organism. In case of measuring probes suitable for measuring biologically important ions particularly significant are electrodes suitable for measuring cations, selected from the group of sodium, potassium, calcium and magnesium. Measuring potassium ion is particularly important in life processes. This explains the widely spread research work concentrated on the elaboration of potassium selective electrodes, on the examination of the properties thereof and optimalization of their preparation. The best potassium ion-selective electrode used so far has been the ion-selective electrode having valinomycine as active ingredient (Swiss Patent Specification No. 479,870).
For biological application the most advantageous property of the electrode is its selectivity factor related to sodium ions: (K.sub.K.sup.Pot, Na) is about 3.times.10.sup.-4. When testing the electrode, it could be observed that the valinomycine based potassium electrode has a higher or equal selectivity towards alkali metal ions of great volume (Rb.sup.+, Cs.sup.+) than towards K.sup.+ i.e. the electrode measures said ions better or with nearly same selectivity as potassium. (L. A. R. Pioda, V. Stankova and W. Simon Anal. Letters 2, 1969, 665). As alkali metal ions often occur together in nature, such coincidence of the selectivity factors can be disadvantageous in case of a potassium electrode prepared for other than biochemical purposes.
The above disadvantages can be eliminated without considerable reduction of the selectivity related to the other ions when using bis crown compounds linked by an aliphatic chain as ion selective substance. Kimura et al (Kimura K., Maeda T., Tamura H., Shono T.,: J. Electroanal. Chem. 95, 1979, 91-101, Kimura K., Tamura H., Shono T.: Bull. Chem. Soc. Jpn. 53 547-548, 1980) disclosed two such compound groups and electroanalytical data thereof. These compounds have a common structural element: two benzo-15-crown-5-units, linked up with dicarboxylic acids through oxygen or nitrogen atoms. Practical applicability of these compounds cannot be estimated, as electroanalytical assay of the compounds is not disclosed in the above articles, only selectivity data related to alkali metal ions are included.